1. Field of the Invention
This invention relates to apparatus for disconnecting coiled tubing from a tool in a well, and more particularly, to a hydraulic disconnect with reduced stress concentrations and bending moment applied to the components thereof.
2. Description of the Prior Art
In the operation of oil and gas wells, it is often necessary to perform several downhole operations in the well. Thus, various types of operating tools have evolved to perform these various operations. Previously, wireline was often used to connect the operating tools to equipment above ground and to lower, set and retrieve the operating tools into and from the well.
More recently, coiled tubing has been used in certain applications which has several advantages. For example, coiled tubing does not rely on gravity for setting and retrieving operating tools, but can rather be used to traverse highly deviated, or horizontal, wells. Also, coiled tubing can be inserted more rapidly into the well than jointed tubing and more easily passed through downhole equipment. Coiled tubing can also be used to convey fluids to the operating tool to hydraulically actuate the tool. Further, fluids such as water, foam, paraffin, corrosion inhibitors, spotting acid, cement, and the like, can be conveyed by the coiled tubing to the well for performing various functions including washing, cleaning and the like.
It is not an uncommon occurrence for the operator to wish to leave a tool in the well or for a tool to become stuck or jammed in the well. In either case, it is necessary to disconnect the coiled tubing from the tool so that the coiled tubing can be removed from the well. The operating equipment is subsequently fished out of the well. To accommodate this, disconnect or emergency release devices have been developed. Such disconnects are installed near the end of the tubing adjacent to the tool at the lower end of the tubing. One such device is shown in U.S. Pat. No. 5,146,984, assigned to a related company to the assignee of the present invention. This device releasably connects the tubing to the downhole tool and permits flow of fluid from the tubing to the tool as well. A prop in the disconnect can be hydraulically actuated to release lugs to disconnect the upper portion of the disconnect from the lower portion and thereby release the coiled tubing from the downhole tool.
Coiled tubing drilling operations put severe stress cycles on the bottom hole assembly in a very short time. With downhole motors that rotate from 180 to 1,000 rpm, thus applying torque to the tubing, the stress cycles required to induce failure in a tool can be short as a few hours. One of the most failure-prone components in a drilling bottom-hole assembly is the hydraulic disconnect.
Many hydraulic disconnects, such as the prior art device described above, use a set of rectangular lugs that rest on an internal shoulder in the bottom sub of the disconnect as a means of supporting tensile load. The lugs are disposed in rectangular-shaped windows which are usually machined radially into the side of the upper sub of the disconnect. The shape of these windows tends to present a problem because of the stress risers of the corner of the windows.
Another problem with many hydraulic disconnects is the location of the windows. When the motor in the bottom hole assembly is rotated, the hydraulic disconnect transmits torque to the tubing so that the drill bit will rotate with respect to the stationary tubing. During drilling, the bottom hole assembly torques up so that the drill bit will start to cut. When the drill actually cuts, the torque is momentarily released. The process is repeated over and over during drilling. Because the cutting operation is not even, these continuous cycles of cutting and releasing result in corresponding cycles of the application of torque followed by the release of the torque.
The torque is transmitted through the hydraulic disconnect at a torque transmission area. The windows in which the lugs in the hydraulic disconnect are disposed are frequently located above the torque transmission area. Thus, torque is applied at the windows which increases the possibility of failure.
When the bottom hole assembly is torqued, there is enough flexibility in the tubing that the top of the disconnect attached to the end of the tubing is pushed toward the side of the well as a result of bending in the tubing. Thus, a bending moment is applied to the hydraulic disconnect. This cyclic bending varies with the torque cycling. The location of the windows above the torque transmission area results in this bending force being applied across the windows.
The torquing and bending results in stress cycling across the window which can result in a fatique cracking problem. The present invention solves this problem by locating the windows below the torque transmission area. Thus, no torque and no bending moment and associated cyclic stresses are applied to the windows.
A further problem with existing hydraulic disconnect design relates to the method of transmitting torque in the tool. Some disconnects, such as that described in U.S. Pat. No. 5,146,984 described above, used interconnected splines which are machined in the top and bottom subs. Other designs use machined fingers that fit in matching notches. Other similar designs exist, but generally these present problems with fretting or fatigue cracking due to the reverse stress cycling that is inherent to drilling bottom hole assemblies.
The present invention solves these problems by using a different approach to transmitting torque. The present invention uses an internal thread in a lower housing portion of the disconnect that is threadingly engaged with a splined ring which is held onto an upper housing portion by a bushing and specially designed lugs. The lugs are shaped in a manner to reduce stress concentrations around lug windows in the upper housing portion. As previously mentioned, the lug windows are located near the lower end of the upper housing portion below the torque transmission area, thereby eliminating torque loading and the bending moment across the windows. This results in a reduction in fatigue problems.
The threading engagement of the splined ring allows the splined ring to tighten the bushing against the lugs. Thus, any slack or looseness between the parts is eliminated, and fretting and other wear problems are minimized because the parts do not chatter against one another while drilling. Of course, this increases the tool life.